Almost there
Overall it's a great game. Music, responsive controls, power ups, etc.
There are, however, two things that I think would make this game better:
1.Hit detection-As in if you get it hit then something happens such as the screen flashing or shaking to tell me that I've been hit.Often I'd find myself playing unaware of how low my health is and suddenly I'm dead and I don't even know what hit me.
2.Make more power ups and make them stick-I think there should be a better variety of power ups and some of them should stay on with you as you play the game, such as, upgrading your firing power. It often gets difficult when your using a level 1 cannon fighting submarines and battleships while trying to find a power up.
If you make some changes like so I think it would be a much better game though some of the beauty in this game lies in the simplicity of it.
Other than that great game and I'm looking forward to your next submission.